Notes adapted from: Coulouris: Distributed Transactions,

95-702 Distributed Systems• Learning Objectives• Understand locks and be able to recognize when locks

are required• Be aware of deadlocks and deadlock mitigation• Understand the importance of transaction processing

systems and a TP system’s multi-tiered architecture• Be able to describe two phase locking (2PL)• Be able to describe the two phase commit protocol (2PC)• Understand how system availability trades off with system

consistency as described by the CAP theorem

95-702 Transactions 1

Transaction Processing (TP) Systems

• Historically, one of the first was American Airlines SABRE – Semi- Automated Business Research Environment - 83,000 transactions per day (1960’s)

• Became IBM’s Airline Control Program• Became IBM’s Transaction Processing Facility (TPF)• Many such modern systems exist:• Oracle Tuxedo (Thousands of transactions per second)• IBM’s Customer Information Control System (CICS)• Most databases and messaging systems provide for transactional support• JEE and Microsoft .NET both provide extensive capabilities for creating and deploying TP applications

TP System Architecture

Adapted From "Principles Of Transaction Processing" Bernstein and Newcomer

User device Front end Program takes requests from the user device

Request Controllerselects the proper transaction to run

The Transaction Server executes the required activities

DatabaseThis represents a multi-tiered TP application.The user-device might be a gas pump or retail sales terminal or browser.

95-702 Transactions 495-702 Transactions 495-702 Transactions 4

Transactions (ACID)• Atomic: All or nothing. No intermediate states are visible. No possibility that

only part of the transaction ran. If a transaction fails or aborts prior to committing, the TP system will undo the effects of any updates (will recover). We either commit or abort the entire process. Checkpointing and Logging and recoverable objects can be used to ensure a transaction is atomic with respect to failures.

• Consistent: system invariants preserved, e.g., if there were n dollars in a bank before a transfer transaction then there will be n dollars in the bank after the transfer. This is largely in the hands of the application programmer.

• Isolated: Two transactions do not interfere with each other. They appear as serial executions. This is the case even though transactions may run concurrently. Locking is often used to prevent one transaction from interfering with another.

• Durable: The commit causes a permanent change to stable storage. This property may be obtained with log-based recovery algorithms. If there has been a commit but updates have not yet been completed due to a crash, the logs will hold the necessary information on recovery.

Assume concurrent visits

private double balance; public synchronized void deposit(double amount) throws RemoteException { balance = balance + amount; } public synchronized void withdraw(double amount) throws RemoteException { balance = balance – amount; }

If one thread invokesa method it acquires a lock. Another threadwill be blocked untilthe lock is released.

This is all that is required for many applications. ButTP middleware must do much more.

What happensif we don’t synchronize?

Communicating Threads (1)Consider a shared queue and two operations: synchronized first() { if Q is empty return false else remove and return front } synchronized append() { add to rear } Is this sufficient? No. If the queue is empty the client of first() willhave to poll on the method. What’s wrong with polling?It is also potentially unfair. Why?

Communicating Threads (2)

Consider again the shared queue and twooperations: synchronized first() { if queue is empty call wait() remove from front } synchronized append() { adds to rear call notify() }

When threads can synchronizetheir actions on an object by meansof wait and notify, the server holdson to requests that cannot immediately be satisfied and theclient waits for a reply untilanother client has produced whatever they need.

Note that both methods are synchronized. Only one thread ata time is allowed in.

This is a simple example. Wait/notify gets tricky fast.

Back to Transactions• A client may require that a sequence of separate requests to a single

server be isolated and atomic. - Isolated => Free from interference from other concurrent clients. - Atomic => Either all of the operations complete successfully or they have no effect at all in the presence of server crashes. - We also want serializability => If two transactions T1 and T2 are running, we want it to appear as if T1 was followed by T2 or T2 was followed by T1. - But, interleaving may have occurred (we like interleaving for performance reasons).

Assume each operation on the server is synchronized - Happy

CaseClient 1 Transaction T;a.withdraw(100);b.deposit(100);c.withdraw(200);b.deposit(200);

Client 2 Transaction W;total = x.getBalance();total = total + y.getBalance();total = total + z.getBalance();

Why are we isolated?

Suppose both run to completion (no partial execution)=> atomic.

Assume each operation on the server is synchronized

Client 1 Transaction T;a.withdraw(100);b.deposit(100);c.withdraw(200);b.deposit(200);

Client 2 Transaction W;total = a.getBalance();total = total + b.getBalance();total = total + c.getBalance();

Are we isolated?

Suppose both run to completion (no partial execution)=> atomic.

Client 1 Transaction T;a.withdraw(100);b.deposit(100);c.withdraw(200);b.deposit(200);

Client 2 Transaction W;total = a.getBalance();total = total + b.getBalance();total = total + c.getBalance();Inconsistent retrieval!

Client 1 Transaction T;bal = b.getBalance(); b.setBalance(bal*1.1);

Client 2 Transaction W;bal = b.getBalance();b.setBalance(bal*1.1);

But are we isolated?

Suppose both run to completion with no partial execution => Atomic.

Client 1 Transaction T;bal = b.getBalance()b.setBalance(bal*1.1);

Client 2 Transaction W;bal = b.getBalance();b.setBalance(bal*1.1);

Lost Update!

Transaction T;a.withdraw(100);b.deposit(100);c.withdraw(200);b.deposit(200);

The aim of any server thatsupports transactions is to maximize concurrency. So,transactions are allowed to execute concurrently if they would have the same effect as serial execution.

Each transaction is created and managed by a coordinator.

Locking is the most popularmechanism to achieve transactionIsolation.

Interacting with a coordinator

Transaction Ttid = openTransaction(); a.withdraw(tid,100); b.deposit(tid,100); c.withdraw(tid,200); b.deposit(tid,200);closeTransaction(tid) orabortTransaction(tid)

Coordinator Interface: openTransaction() -> transID closeTransaction(transID) -> commit or abort abortTransaction(TransID)

Serially Equivalent• For two transactions to be serially equivalent, it is

necessary and sufficient that all pairs of conflicting operations of the two transactions be executed in the same order at all of the objects they both access. (Coulouris)

• Let r1(x) mean that transaction 1 reads x.

• Let w2(x) mean that transaction 2 writes x.

• r1(x) r2(x) do not conflict (may be ordered either way).

• r1(x) w2(x) do conflict (order matters)

• w1(x) w2(x) do conflict (order matters)

Locking to Attain Serializability• With locks, each transaction reserves access to the data it

uses. • There are read locks rL1(x) and write locks wL2(x).• Before reading, a read lock is set. Before writing, a write

lock is set.• A transaction can obtain a lock only if no other transaction

has a conflicting lock on the same data item.• Locks may be removed with wU1(y) or rU1(x)• In the next two slides, we return to the earlier examples

and apply this locking scheme. They become serially equivalent.

Allow either one to run first - Two Phase Locking for Concurrency Control

Client 1 Transaction T;Get write lock on aa.withdraw(100);Get write lock on bb.deposit(100);Get write lock on cc.withdraw(200);b.deposit(200);Unlock a,b,c

Client 2 Transaction W;Get a read lock on atotal = a.getBalance();Get a read lock on btotal = total + b.getBalance();Get a read lock on ctotal = total + c.getBalance();Unlock a,b,c

Allow either one to run first - Two Phase Locking for Concurrency Control

Client 1 Transaction T;Get a read lock on bbal = b.getBalance()Upgrade to writeb.setBalance(bal*1.1);Unlock b

Client 2 Transaction W;Get a read lock on bbal = b.getBalance();Upgrade to write lockb.setBalance(bal*1.1);Unlock b

Locking is not enough

• Transaction T1 Transaction T2 rL1(x)

rU1(x)

wL1(y)

wU1(y)95-702 Transactions 20

rL2(y)r2(y)wL2(x)w2(x)rU2(y)wU2(x)

What pairs are in conflict?• Transaction T1 Transaction T2 rL1(x)

rU1(x)

wL1(y)

wU1(y)

“all pairs of conflicting operations of the two transactions be executed in the same order”Coulouris

What pairs are in conflict?

rU1(x)

wL1(y)

To be serially equivalent: If r1(x) occurs before w2(x)then w1(y) mustoccur before r2(y) andIf r1(x) occurs after w2(x)then w1(y) mustoccur after r2(y).

rU1(x)

wL1(y)

Locking alone doesnot enforce the ruleson serially equivalence.

Locking is not enough• Transaction T1 Transaction T2 rL1(x)

rU1(x)

wL1(y)

wU1(y)

We now have r1(x), r2(y), w2(x), w1(y). But we need…

either T1 followed by T2 or T2 followed by T1.

We need either r1(x), w1(y), r2(y), w2(x) orr2(y), w2(x), r1(x), w1(y).

How do we guarantee that?

Two phase locking (2PL) demands that all locks are obtained for each transaction before releasing any of them!

Lock and Unlock in two Phases• Transaction T1 Transaction T2 rL1(x)

wL1(y)

wU1(y)

rU1(x)

Now T1 and T2are serialized.This may leadto deadlock. Theserialization proofexists but is beyond course scope.

What might Lock_Item() look like?

Lock_Item(x) B: if(Lock(x) == 0) Lock(x) = 1 else { wait until Lock(x) == 0 and we are woken up. GOTO B }Now, a transaction is free to use x.

Not interleaved with othercode until this terminates orwaits. In java, this would be a synchronized method.

Similar to the code abovethat used a shared queue.

95-702 Transactions 28Master of Information System Management

And unlock_item() ?The transaction is done using x.

Unlock_Item(x) Lock(x) = 0 if any transactions are waiting then wake up one of the waiting transactions. Not interleaved with other

code. If this were java, thismethod would be synchronized.

95-702 Transactions 29Master of Information System Management

Does this allow for any concurrency?

Transaction T1 Transaction T2

Lock_Item(x) Lock_Item(y) T1 uses x T2 uses y

Unlock_Item(x) Unlock_Item(y)If x differs from y these two transactions proceed concurrently.If both want to use x, one waits until the other completes.

In reality, thecoordinatorwould do the locking.

Locks May Lead to Deadlock

Four Requirements for deadlock:

(1) Resources need mutual exclusion. They are not thread safe. (2) Resources may be reserved while a process is waiting for more. (3) Preemption is not allowed. You can't force a process to give up a resource. (4) Circular wait is possible. X wants what Y has and Y wants what Z has but Z wants what X has.

Solutions (short course):

Prevention (disallow one of the four) Avoidance (study what is required by all before beginning) Detection (using time-outs or wait-for graphs) and recovery

Deadlock

Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Local Transactions (Single Server)

• Typically handled directly by a database.• Call beginTransaction on an SQLConnection object.• Exceute SQL statements.• Call commit or rollback.• Locks may be held on the rows/tables involved.• Everything is done on a copy until the commit or

rollback.• Deadlock may be detected with wait-for graphs.• In distributed transactions, deadlock may be detected

with time-outs.

Transactions On Objects (Single Server)

“A”

“B”

“C”

Lock management, recovery managementand traditionalmiddleware

a =lookUp(“A”)b =lookUp(“B”)beginTran x = a.read() b.write(x)closeTran orabortTran

a =lookUp(“A”)beginTran x = a.read()closeTran orabortTran

Recoverable objects

95-702 Transactions 3495-702 Transactions 34

What is a Recoverable Object?

A recoverable object follows the Golden Rule of Recoverability:

“Never modify the only copy.”

The transaction make changesto local copies of resources until a commit or rollback.

When the server is running it can keep allof its objects in itsvolatile memory and records its committed objects in a recoveryfile.

Upon recovery, the server can restore theobject’s latest committed versions.

If a transaction fails, only thetentative versions of the objectshave changed, not the non-volatile copy.

Distributed Transactions (More than one server)

Begin transaction BookTrip book a plane from Qantas book hotel from Hilton book rental car from HertzEnd transaction BookTrip

The Two Phase Commit Protocol is a classic solution for atomicity and consistency.

Interacting with a coordinator

Transaction Ttid = openTransaction(); a.withdraw(tid,100); b.deposit(tid,100); c.withdraw(tid,200); b.deposit(tid,200);closeTransaction(tid) orabortTransaction(tid)

Coordinator Interface: openTransaction() -> transID closeTransaction(transID) -> commit or abort abortTransaction(TransID)

Think about atomicity andconsistency.

Client Talks to a Coordinator

BookTrip Coordinator

BookPlane Participant

BookHotel Participant

BookRentalCar Participant

Different servers

BookTrip Client

openTrans Unique Transaction IDTID

Recoverable objects neededto book a plane

Recoverable objects neededto book a hotel.

TID = openTransaction()

Recoverable objects neededto rent a car.

Any server

Client Uses Services

Different servers

BookTrip Client

Call + TID

plane.bookFlight(111,”Seat32A”,TID)

Any server

Different servers

BookTrip Client

Participants Talk to Coordinator

The participant knows where the coordinator is because that information can be included inthe TID (eg. an IP address.)The coordinator now has a pointer to the participant.

The participant onlycalls join if it has notalready done so.

join(TID,ref to participant)

Different servers

BookTrip Client

Suppose All Goes Well (1)

OK returned

Different servers

BookTrip Client

Suppose All Goes Well (2)

OK returnedOK returned

OK returned

CloseTransaction(TID) Called

Coordinator begins2PC and this results ina GLOBAL COMMITsent to each participant.

Different servers

BookTrip Client

This Time No Cars Available (1)

OK returned

NO CARS AVAILabortTransaction(TID) called

Different servers

BookTrip Client

OK returned

NO CARS AVAILabortTransaction(TID) called

Coordinator sends a GLOBAL_ABORT to allparticpants

Different servers

BookTrip Client

ROLLBACK CHANGES

OK returned

NO CARS AVAIL abortTransaction(TID)

abortTransaction

Each participantGets a GLOBAL_ABORT

ROLLBACK CHANGES

Different servers

BookTrip Client

BookPlane Server Crashes After Returning ‘OK’ (1)

OK returned

Different servers

BookTrip Client

BookPlane Server Crashes After Returning ‘OK’ (2)

OK returned

Coordinator excutes 2PC:Ask everyone to vote.No news from the BookPlaneParticipant so multicast a GLOBAL ABORT

Different servers

BookTrip Client

ROLLBACK

BookPlane Server Crashes after returning ‘OK’ (3)

OK returned

ROLLBACK

GLOBAl ABORT

ROLLBACK

Two-Phase Commit Protocol

BookTripCoordinator

BookPlane

BookHotel

BookRentalCarPhase 1 BookTrip coordinator sends a Vote_Request to each process. Each process returns a Vote_Commit or Vote_Abort.

Vote_Request

Vote Request

Vote_Commit

Vote Commit

Two-Phase Commit Protocol

BookTripCoordinator

BookPlane

BookHotel

BookRentalCarPhase 2 BookTrip coordinator checks the votes. If every process votes to commit then so will the coordinator.In that case, it will send a Global_Commit to each process. If any process votes to abort the coordinator sends a GLOBAL_ABORT.Each process waits for a Global_Commit message before committing its part of thetransaction.

Global Commit

Global CommitACK

Global Commit

2PC Finite State Machine from Tanenbaum

Abort Commit

Commit----------Vote-request

Vote-commit----------------Global-commit

Vote-abort--------------Global-abort

AbortCommit

Vote-request-----------------Vote-commit

Vote-request-----------------Vote-abort

Global-commit-------------------ACKGlobal-abort

----------------ACK

State has already been saved to permanent storage.

BookTrip Coordinator Participant

2PC Blocks in Three Places

Abort Commit

AbortCommit

----------------ACK

If waiting too long for a Vote-Requestsend a Vote-Abort

Abort Commit

AbortCommit

Vote-request-----------------Vote-abort Global-commit

-------------------ACKGlobal-abort

----------------ACK

If waiting too long After Vote-requestSend a Global-Abort

Abort Commit

AbortCommit

----------------ACK

If waiting too long we can’t simply abort! We must waituntil the coordinator recovers. We might also make queries on other participants.

Abort Commit

AbortCommit

----------------ACK

If this process learns that another has committed then this process is free to commit. The coordinator must have sent out a Global-commit that did not get to this process.

Abort Commit

AbortCommit

----------------ACK

If this process learns that another has aborted then it too is free to abort.

Abort Commit

AbortCommit

----------------ACK

Suppose this process learns that another process is still in its init state. The coordinator must havecrashed while multicasting the Vote-request. It’s safe forthis process (and the queried process) to abort.

Abort Commit

AbortCommit

----------------ACK

Tricky case: If the queried processes are all still in their ready state what do we know? We have to block and wait until theCoordinator recovers.

Summary 2PL and 2PC• Two phase locking (2PL) is a concurrency control

protocol – guarantees transactions have the same effect as serial execution. Guarantees transactions do not interfere with each other. Does it prevent deadlock?

• The two phase commit protocol (2PC) is an atomic commitment protocol – a special type of consensus protocol. Does it prevent deadlock? Typically used for distributed transactions.

• A consensus protocol tries to reach agreement in the presence of crashes or failures. All agree?

DS Principles For Internet Scale applications(Ian Gorton SEI)

• Goal: highly available and highly scalable.• Principle: Complex systems do not scale. 2PC is complex.

Thus, on the internet, weak consistency is replacing strong consistency. For example, a change to a Google Doc may not be available to all immediately.

• Principle: Statelessness. Any server, any time implies keeping state off the server. No routing to correct server.

• Principle: Allow failures but be able to monitor system behavior.

• The CAP Theorem was first announced in 2000.• Principle: Mixed approaches (consistency tradeoffs) are

possible.

The CAP Theorem (Brewer)• Consistency: if a value is written to node 1 then it is that

value that is read from node 2. It is not the case that one node has an old value and another node has the most recent value. Brewer says “consistency (C) is equivalent to having a single up-to-date copy of the data;”

• Available: The system is highly available for updates.• Partition tolerance: If a network failure occurs the system

will still work.• CAP Theorem: You may only have a system with two of

these three. You may have either CA, CP, or AP.

The Traditional CAP Theorem (Brewer)

• Essentially, the CAP theorem says that in the face of network partitions, you may either have availability or consistency but not both.

• Example: Suppose there is a break in the network between an automated teller machine and the main banking database. We have a choice. We can be either unavailable for ATM use or we can allow for small transactions and be a little inconsistent. If we choose the latter we can still reconcile any inconsistencies later.

• Example: Suppose we are using HTML5 local storage and do some disconnected work offline. We are choosing availability over consistency – and may only require eventual consistency.

CAP Theorem Update

Brewer says (2012):

“Because partitions are rare, CAP should allow perfect C and A most of the time, but when partitions are present or perceived, a strategy that detects partitions and explicitly accounts for them is in order. This strategy should have three steps: detect partitions, enter an explicit partition mode that can limit some operations, and initiate a recovery process to restore consistency and compensate for mistakes made during a partition.”

Notes adapted from: Coulouris: Distributed Transactions,

Documents

Transcript of Notes adapted from: Coulouris: Distributed Transactions,

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